This invention relates to an auto tuning/storing receiver using a phase lock loop frequency synthesizer.
A receiver using a PLL frequency synthesizer is generally designed such that the division ratio of the PLL circuit is varied through a scan circuit and the scanning of tuning frequency by the scan circuit is discontinued in response to the output of an IF amplifier or a detector to thereby enable auto tuning. An improvement in the above-mentioned type of radio receiver has already been proposed which includes a memory circuit for storing division ratios of the receiving frequencies or an information characteristic of such division ratios (referred to as tuning signals hereinafter), i.e. radio receivers capable of storing receivable frequencies. See, for example, Japanese Patent Kokoku No. 52/32805. This sort of receivers however requires that the listener push a switch or the like to place the instantaneous receiving frequency into a memory circuit whenever the receiver receives a properly broadcast signal of a certain station and stops a scanning of the receiving frequency.
A further improved receiver has be devised which eliminates the listener's manual operation and in particular loads the instantaneous receiving frequency into the memory circuit automatically each time it achieves auto tuning through scanning and restarts the scanning of the receiving frequency after the loading of such a receivable frequency (this is referred to as an "auto tuning/storing receiver"). This receiver design offers significant advantages especially in automobile receivers, that is a minimum of required listener's interactions and a maximum of driving safety.
The capacity of the memory circuit or the number of tuning frequencies to be stored therein is inevitably limited so that criteria shall be established as to whether the instantaneous tuning frequency is to be stored in the memory circuit. The problem is where the reference level shall be set as to the field intensity of a monitoring broadcast. In other words, when provided that the reference level of field intensity (this is referred to as "storing criterion" hereinafter) is selected low, the receiver exhibits high sensitivity for almost all of the broadcasts and therefore presents the problem that the memory circuit becomes filled with only receivable frequencies in a low frequency range shortly after the scanning of frequency has been started. On the other hand, if the reference level is too high, then the broadcasts which are allowed to be stored in the memory circuit are limited to those having very high field intensities and the memory circuit is not used fully and effectively when this occurs.
A possible resolution to circumvent the problems discussed above is to manually re-adjust the storing criterion and repeat a sequence of auto tuning/storing. With automobile receivers, the manual operations required should be reduced as much as possible because of the needs of safe driving. The same applies to other radio receivers.